1. Field of the Invention
The present invention relates to arrays of photosensitive sensor groups, each sensor group comprising vertically stacked sensors (typically, vertically stacked red, green, and blue sensors), and each sensor group in the array positioned at a different pixel location. In each sensor group, semiconductor material chromatically filters incident electromagnetic radiation vertically (optionally, other material also filters the radiation) and each sensor simultaneously detects a different wavelength band.
2. Background of the Invention
The term “radiation” is used herein to denote electromagnetic radiation.
The expression “top sensor” (of a sensor group) herein denotes the sensor of the group that radiation, incident at the sensor group, reaches before reaching any other sensor of the group. The expression that the sensors of a sensor group are “vertically stacked” denotes that one of the sensors is a top sensor of the group, and that the group has an axis (sometimes referred to as a “vertical axis”) that extends through all the sensors. A vertical color filter (“VCF”) sensor group typically includes vertically stacked sensors configured such that the group's top sensor has a top surface that defines a normal axis (e.g., is at least substantially planar), and when radiation propagating along a vertical axis of the group is incident at the group, the radiation is incident at the top sensor with an incidence angle of less than about 30 degrees with respect to the normal axis (e.g., the radiation is normally incident at the group). Typically, a VCF sensor group consists of three vertically stacked sensors: a blue sensor, a green sensor, and a red sensor.
An array of VCF sensor groups defines an imaging plane. Typically, each VCF sensor group of an array has a top sensor having a top surface that defines a normal axis, the top surfaces of the array's sensor groups are parallel to each other, and the array's imaging plane has a normal axis (the “array's vertical axis”) that is parallel to the normal axis of each group.
The expression used herein that two elements, included in a structure having a vertical axis, are “laterally” (or “horizontally”) separated denotes that there is an axis parallel to the vertical axis that extends between the elements but intersects neither element. The expression that an item “comprises” an element is used herein (including in the claims) to denote that the item is or includes the element.
The “size” of each sensor of a VCF sensor group is the area (projected in a plane perpendicular to a vertical axis of the group) of the sensor's carrier-collection element. The expression “minimum-sized” sensor of a VCF sensor group herein denotes each sensor of the group whose carrier-collection element has an area (projected in a plane perpendicular to a vertical axis of the group) that is less than or equal to the area (projected in the same plane) of the carrier-collection element of each other sensor of the group.
The expression “pixel sensor location” within an array of VCF sensor groups herein denotes the location of one of the VCF sensor groups. The location of the relevant VCF sensor group is determined in an appropriate way given the sensor group's configuration. For example, when the sensors of a VCF sensor group have different sizes and the carrier-collection element of each of the group's sensors has an optical center, the group's location may be the area-weighted average of the positions of projections of the optical centers of group's sensors on a plane perpendicular to a vertical axis of the group. Or, the location of a VCF sensor group in an array may be the area-weighted average of the positions of projections of the optical centers of group's minimum-sized sensors on a plane perpendicular to a vertical axis of the group (or the position of the projection of the optical centers of group's minimum-sized sensor on such plane, if the group has only one minimum-sized sensor). Thus, in an array of VCF sensor groups comprising a green layer (comprising 4N green sensors, where N is a number), a blue layer (comprising N blue sensors), and a red layer (comprising N red sensors), with all green sensors having the same size, each blue sensor and red sensor having size four times that of each green sensor, and each blue sensor and red sensor shared by four VCF sensor groups, the array would have 4N pixel sensor locations.
MOS active pixel sensors are known in the art. Multiple-wavelength band arrays of active pixel sensors are also known in the art. One type of multiple-wavelength band active-pixel-sensor array employs red, green, and blue sensors disposed horizontally in a pattern at or near the semiconductor surface. Color overlay filters are employed to produce the color selectivity between the red, green, and blue sensors. Such sensors have the disadvantage of occupying a relatively large area per resolution element as these sensors are tiled together in a plane. In addition, reconstruction of a color image from such a sensor array is computationally intensive and often results in images with artifacts, defects, or inferior resolution.
Several types of VCF sensor groups and methods for fabricating them are described in U.S. Pat. No. 6,727,521, issued Apr. 27, 2004, and in U.S. Pat. No. 6,864,557, issued on Mar. 8, 2005. A VCF sensor group includes at least two photosensitive sensors that are vertically stacked with respect to each other (with or without non-sensor material between adjacent sensors). Each sensor of a VCF sensor group has a different spectral response. Typically, each sensor has a spectral response that peaks at a different wavelength. In some embodiments, a VCF sensor group (or one or more of the sensors thereof) includes a filter that does not also function as a sensor.
A VCF sensor group simultaneously senses photons of at least two wavelength bands in the same area of the imaging plane. In contrast, time sequential photon sensing methods do not perform photon sensing at the same time for all wavelength bands. The sensing performed by a VCF sensor group included in an imager occurs in one area of the imager (when the imager is viewed vertically), and photons are separated by wavelength as a function of depth into the sensor group.
Typically, each sensor detects photons in a different wavelength band (e.g., one sensor detects more photons in the “blue” wavelength band than each other sensor, a second sensor detects more photons in the “green” wavelength band than each other sensor, and a third sensor detects more photons in the “red” wavelength band than each other sensor), although the sensor group typically has some “cross-talk” in the sense that multiple sensors detect photons of the same wavelength.
VCF sensor groups can be used for a variety of imaging tasks. In preferred embodiments, they are used in digital still cameras. However they can be employed in many other systems, such as linear imagers, video cameras and machine vision equipment.
A VCF sensor group uses the properties of at least one semiconductor material to detect incident photons, and also to selectively detect incident photons of different wavelengths at different depths in the group. The detection of different wavelengths is possible due to the vertical stacking of the sensor layers of the sensor group in combination with the variation of optical absorption depth with wavelength in semiconductor materials. The costs of manufacturing VCF sensor groups are substantially reduced because VCF sensor groups do not require external color filters (as are traditionally used in color image sensors) and do not require color filters that are distinct from the sensors themselves (the sensors themselves are made of semiconductor material that itself provides a filtering function). However, in some embodiments of the invention, VCF sensor groups do include (or are used with) color filters that are distinct from the sensors themselves. The spectral response characteristics of VCF color sensor groups typically are much more stable and less sensitive to external factors such as temperature or other environmental factors (that may be present during or after manufacturing) than are conventional color sensors with non-semiconductor based filters.
A VCF sensor group is preferably formed on a substrate (preferably a semiconductor substrate) and comprises a plurality of vertically stacked sensors configured by doping and/or biasing to collect photo-generated carriers of a first polarity (preferably negative electrons). The sensors include (or pairs of the sensors are separated by) one or more reference layers configured to collect and conduct away photo-generated carriers of the opposite polarity (preferably positive holes). The sensors have different spectral sensitivities based on their different depths in the sensor group, and on other parameters including doping levels and biasing conditions. In operation, the sensors are individually connected to biasing and active pixel sensor readout circuitry. VCF sensor groups and methods for fabricating them are discussed more fully in above-referenced U.S. Pat. Nos. 6,727,521 and 6,864,557.
Arrays of VCF sensor groups, such as some of those described in above-cited U.S. patent application Ser. Nos. 10/738,484 and 10/355,723, have important advantages over arrays of single-layer sensors (e.g., conventional arrays having a Bayer pattern, and arrays that implement conventional color-filter-mosaic technologies), including the advantages of measuring luminance at every pixel sensor location and measuring color in a way that does not induce color aliasing artifacts. The additional advantage of measuring color at every pixel sensor location to achieve good chroma resolution is less important and can be sacrificed by implementing one layer (e.g., the green, red, or blue layer) of an array of VCF sensor groups to be readable with (to “have”) full resolution and at least one other layer of the array to have lower resolution (e.g., as described in above-cited U.S. patent application Ser. Nos. 10/738,484, and 10/355,723).
For example, the green layer of an array of VCF sensor groups can be implemented to have with full resolution, the outputs of clusters of sensors in the blue layer can be combined to implement the blue layer with less than full resolution, and the outputs of clusters of sensors in the red layer can be combined to implement the red layer with less than full resolution. Such an array in which all the sensors have substantially the same size, the top layer is the blue layer, the green layer is between the blue layer and the red layer, each cluster of sensors in the blue layer includes four sensors (e.g., is a two sensor by two sensor cluster), and each cluster of sensors in the red layer includes four sensors (e.g., is a two sensor by two sensor cluster), is referred to herein as a “1-4-1” array (or an array having “1-4-1” organization) to indicate that the resolution of the green layer is higher by a factor of four than that of each of the red and blue layers.
The term “output” of a sensor (of a VCF sensor group) herein denotes a signal indicative of incident photon intensity at the sensor (for example, a signal indicative of photogenerated charge). The expression that the outputs of sensors of a layer of a VCF sensor group array are “combined” herein denotes that the sensors have distinct, laterally separated carrier-collection elements, and these elements are electrically coupled together (e.g., by conductors deposited on the surface of the array and/or conductors coupled to the array) during readout of the elements.
The notation that an array of VCF sensor groups is an “X-Y-Z” array (or has “X-Y-Z” organization) herein assumes that each sensor group includes a top (“Z”) sensor (typically a blue sensor), an intermediate (“Y”) sensor (typically a green sensor) below the top sensor, and a third (“X”) sensor (typically a red sensor) below the intermediate sensor (and the array consists of a top layer of top sensors, an intermediate layer of intermediate sensors, and a third layer of third sensors), and denotes that the ratio of the top layer's resolution to the intermediate layer's resolution is Z/Y and the ratio of the intermediate layer's resolution to the third layer's resolution is Y/X. Typically, an X-Y-Z array is an array having a top (“Z” or “blue”) layer of blue sensors, an intermediate (“Y” or “green”) layer of green sensors, and a bottom (“X” or “red”) layer of red sensors.
In another 1-4-1 array in which the top layer is the blue layer, and the green layer is between the blue layer and the red layer, all sensors in the green layer have substantially the same size and each sensor in each of the blue layer and red layer has size substantially equal to four times the size of each green layer sensor. Each sensor in each of the blue and red layers is shared by four VCF sensor groups in the sense that its carrier-collection element is shared by these four sensor groups.
“1-4-1” arrays of VCF sensor groups have an advantage in that their green channel response is not very far from a theoretically ideal luminance spectral sensitivity curve, and thus they can adequately capture high frequency luminance information while also realizing their implementation advantages relative to 4-4-4 arrays having red, green, and blue sensors of the same size as the green sensors of the 1-4-1 arrays. However, the full-resolution readout of green in such 1-4-1 arrays undesirably requires four separate contacts to the green layer (per each contact to the red layer). Each contact to the red or green layer undesirably occupies much space in the array.
U.S. patent application Ser. No. 10/738,484 discloses an array of VCF sensor groups in which each group includes a blue sensor, a green sensor, and a red sensor. Each of the red sensor and green sensor of each group is larger than the group's blue sensor and is shared with at least one other VCF sensor group. The blue sensors are typically implemented near the top surface of a semiconductor wafer and the red sensors deeper in the wafer. The size of each red sensor is roughly four times the size of each blue sensor, and sets of four adjacent VCF sensor groups share a single red sensor. Each green sensor's size can be about half the size of each red sensor (or can be the same as each blue sensor's size or can be any of a variety of other sizes). An implementation of such an array in which the each red sensor's size is four times the size of each blue sensor, the size of each green sensor is about half the size of each red sensor, the top layer is the blue layer, and the bottom layer is the red layer is a “1-2-4” array (or an array having “1-2-4” organization) in the sense that the resolution of its green layer is higher by a factor of two than that of its red layer and the resolution of its blue layer is higher by a factor of four than that of the red layer.